Subscriber line interface circuits (SLICs) are employed by telecommunication service providers to interface a communication wireline pair with subscriber equipment, including both voice and data communication devices. In order to be interfaced with a variety of telecommunication circuits including circuits providing digital codec functionality, the transmission channels of the SLIC must conform with a very demanding set of performance requirements, including but not necessarily limited to accuracy, linearity, low noise, filtering, insensitivity to common mode signals, low power consumption, and ease of impedance matching programmability. In this regard, the DC voltage parameters of a ringing SLIC are governed by both the operational requirements of the device to which the SLIC is coupled (such as the minimum on-hook voltage (e.g., on the order of 40 VDC) required by a facsimile machine or modem), as well as telecommunication industry safety standards (that currently limit the allowable sustained DC voltage to a value of 56.5 VDC).
A reduced complexity illustration of a conventional multi-current control-based circuit amplifier architecture for complying with this requirement is diagrammatically illustrated in FIG. 1, as comprising a (Tip/Ring) amplifier 10 having its non-inverting (+) input 11 coupled to a voltage dividing node 21 of a voltage divider 20. The voltage divider is formed of a pair of equal valued (R) resistors 22 and 23, that connect a DC battery voltage (VBAT) to ground (GND). The amplifier 10 has an inverting (−) input 12 coupled to an output node 13 by way of a feedback (value R) resistor 14. The inverting (−) input 12 of the amplifier is further coupled to a current source 31, which may be configured as a current mirror, and is operative to supply a current corresponding to that sensed flowing through the voltage divider 20, or I=VBAT/2R.
In order to constrain the amplifier input voltage within prescribed operational limits (e.g., the above referenced 56.5 VDC value) irrespective of the value of the battery voltage VBAT, the inverting (−) input 12 of amplifier 10 is further coupled to a plurality of current source/sink circuits 32 and 33. The current mirror (sink) 32 sinks an equal and opposite polarity current I=VBAT/2R from the inverting polarity (−) input node 12, so that current source/sink pair 31/32 effectively provide a pair of currents at the inverting (−) input node 12 that are complementary to those provided at the non-inverting (+) input node 11, by way of the voltage divider 20. An additional current mirror 33 is used to controllably supply the amplifier's inverting polarity (−) node 12 with an auxiliary, compensation current derived in accordance with MTU specifications and designated in FIG. 1 as current I=Vmtu/R.
Typically, this auxiliary current is generated by sensing the current through resistors 22 and 23, and then comparing the sensed current to a threshold current reference value. The difference between these two currents is applied to current mirror 33, which produces the auxiliary current I=Vmtu/R. Unfortunately, such a multi-current source based regulation scheme not only dissipates substantial power, but is prone to introducing voltage regulation component-based noise into the voice path of the SLIC.